drm/exynos: fix gem buffer allocation type checking

This patch fixes gem buffer allocation type checking.
EXYNOS_BO_CONTIG has 0 so the checking should be fixed
to 'if (!(flags & EXYNOS_BO_NONCONTIG))'

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>